1. Field of the Invention
The present invention relates to an R.F. amplifier circuit, and more particularly to an inexpensive R.F. amplifier circuit having a high quality factor Q for tuning frequency and an excellent characteristic for a high electric field strength.
In general, most important factors in measuring the performance of a radio receiver are sensitivity, selectivity, fidelity and output power. From those factors, an overall performance of the radio receiver is evaluated. Of those factors, the selectivity is an index to show ability of eliminating interference from undesired broadcasting waves and selecting only a desired broadcasting wave. Recently, since a number of broadcasting waves are radiated and the sensitivity of the radio receiver has been improved to enable the reception of a weak broadcasting wave, the chance of interference by the broadcasting waves having close frequencies has increased. The selectivity mainly depends on a performance, type and number, of tuning circuits. Generally, the higher the Q of the tuning circuit, and the greater the number of the circuits included, the higher is the selectivity.
The present invention is directed to an R.F. amplifier circuit having an excellent selectivity, which does not need to increase the number of circuits and can improve the Q of the tuning circuit with a simple and inexpensive circuit.
2. Description of the Prior Art
FIG. 1 shows a conventional R.F. amplifier circuit in which a bias to a transistor 4 is fed from a power supply terminal 3 through a resistor 13 in order to eliminate the influence of a bypassing capacitor 12 to an LC tuning circuit 11. Because of the limitation of the bias current supplied through the resistor 13, an optimum biasing current or a collector voltage of the transistor 4 for a cross modulation is not supplied. Accordingly, the conventional circuit of FIG. 1 has a drawback of interference at a high electric field strength and cross modulation.
Assuming that an output impedance of the transistor 4 is Rt, a capacitance of a capacitor 9 is C.sub.1, an inductance of an inductor 10 is L.sub.1, and reactances of a coupling capacitor 14 and the bypassing capacitor 12, i.e. 1/.omega.WC.sub.3 and 1/.omega.WC.sub.2, are sufficiently small, an equivalent circuit of the tuning circuit of FIG. 1 can be represented as shown in FIG. 2. A quality factor .theta..sub.1 of the tuning circuit is given by; ##EQU1##
As a method for improving the quality factor .theta..sub.1 of the tuning circuit of FIG. 1, a variable inductor 15 of the LC tuning circuit 11 may be tapped to step up the output impedance of the transistor 4, as shown in FIG. 3. However, in general, it is very difficult and expensive to provide a tap to such a variable inductor (R.F. coil).